feat: Phase 2 – Budget-Engine + Direktzugriff (http://IP:Port)

Budget:
- Modelle + Migration 0003: budget_template_lines, planned_income_lines,
  budget_periods, budget_period_lines.
- services/budget: Perioden-Grenzen (einstellbarer Starttag, Label nach Endmonat),
  Standard-Vorlage vs. Monats-Override (Snapshot-on-Customize), Soll/Ist inkl.
  budgetierter Sparen-Transfers, geplante Einnahmen (Soll/Ist).
- routers/budgets: Vorlage, geplante Einnahmen (CRUD), Monats-Soll/Ist,
  Monats-Override, Zurücksetzen. pytest: Perioden, Soll/Ist, Override/Reset, Einnahmen.
- Frontend: Budget-Seite (Monat + Standard-Budget), Fortschrittsbalken grün/rot,
  Monats-Navigation, editierbare Soll-Beträge; Sparen-Option im Transfer-Formular.

Deployment-Fix (Nutzer-Anforderung: http://IP:Port muss funktionieren):
- Dienst bindet an APP_BIND_HOST=0.0.0.0 (direkt im LAN erreichbar).
- APP_COOKIE_SECURE=false als Standard (sonst kein Login über reines HTTP);
  für reinen HTTPS-Betrieb umstellbar. Doku + install.sh entsprechend angepasst.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/models.py

@@ -289,3 +289,91 @@ class ExchangeRate(Base):
     currency: Mapped[str] = mapped_column(String(3), primary_key=True)
     rate_per_eur: Mapped[float] = mapped_column(Numeric(18, 8), nullable=False)
     source: Mapped[str] = mapped_column(String(16), nullable=False, default="ecb")
+
+
+# ---------------------------------------------------------------------------
+# Phase 2: Budget (ein Standard-Budget als Vorlage + Monats-Overrides)
+# Planned-Beträge sind in der Hauptwährung des Nutzers.
+# ---------------------------------------------------------------------------
+
+
+class BudgetTemplateLine(Base):
+    """Standard-Budget je Kategorie (die editierbare Vorlage)."""
+
+    __tablename__ = "budget_template_lines"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    owner_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    category_id: Mapped[int] = mapped_column(
+        ForeignKey("categories.id", ondelete="CASCADE"), nullable=False
+    )
+    planned_cents: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    __table_args__ = (
+        UniqueConstraint("owner_id", "category_id", name="uq_tpl_owner_category"),
+    )
+
+
+class PlannedIncomeLine(Base):
+    """Geplante Einnahme (z. B. Gehalt) mit optionaler Kategorie und Zielkonto."""
+
+    __tablename__ = "planned_income_lines"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    owner_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    label: Mapped[str] = mapped_column(String(120), nullable=False)
+    category_id: Mapped[int | None] = mapped_column(
+        ForeignKey("categories.id", ondelete="SET NULL"), nullable=True
+    )
+    target_account_id: Mapped[int | None] = mapped_column(
+        ForeignKey("accounts.id", ondelete="SET NULL"), nullable=True
+    )
+    planned_cents: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    position: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+
+class BudgetPeriod(Base):
+    """Ein Budget-Monat. Existiert erst, sobald der Monat angepasst wurde."""
+
+    __tablename__ = "budget_periods"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    owner_id: Mapped[int] = mapped_column(
+        ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    label: Mapped[str] = mapped_column(String(7), nullable=False)  # YYYY-MM (End-/Hauptmonat)
+    customized: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)
+
+    lines: Mapped[list[BudgetPeriodLine]] = relationship(
+        back_populates="period", cascade="all, delete-orphan"
+    )
+
+    __table_args__ = (
+        UniqueConstraint("owner_id", "label", name="uq_period_owner_label"),
+    )
+
+
+class BudgetPeriodLine(Base):
+    """Monatsspezifischer Budget-Wert (Snapshot-on-Customize)."""
+
+    __tablename__ = "budget_period_lines"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    budget_period_id: Mapped[int] = mapped_column(
+        ForeignKey("budget_periods.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    category_id: Mapped[int] = mapped_column(
+        ForeignKey("categories.id", ondelete="CASCADE"), nullable=False
+    )
+    planned_cents: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    period: Mapped[BudgetPeriod] = relationship(back_populates="lines")
+
+    __table_args__ = (
+        UniqueConstraint("budget_period_id", "category_id", name="uq_periodline_period_category"),
+    )